The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized Millimeter Wave (mmWave) frequency spectrum between 30 G and 300 G Hz for the next generation broadband cellular communication networks. The available spectrum of mmWave band can be up to hundreds of times greater than the conventional cellular system. The mmWave wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmWave spectrum has wavelengths in the order of millimeter, thus the name. The very small wavelengths of the mmWave spectrum enable large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions.
With recent advances in mmWave semiconductor circuitry, mmWave wireless system has become a promising solution for real implementation. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges for the mmWave network. In general, a cellular network system is designed to achieve the following goals: 1) Serve many users with widely dynamical operation conditions simultaneously; 2) Robust to the dynamics in channel variation, traffic loading and different QoS requirement; and 3) Efficient utilization of resources such as bandwidth and power. Beamforming adds to the difficulty in achieving these goals.
The backhaul link in mmWave backhaul systems has the following characteristics: 1) very high directivity transmission; 2) limited dominant paths (either LOS or NLOS). In principle, beam training mechanism, which includes both initial beam alignment and subsequent beam tracking, ensures that a Hub and Stations that can be served by the Hub are aligned for data communication. At least for initial network access, measurement pilot transmission at the Hub is needed. Consideration for providing measurement pilot opportunities in downlink includes: blanket-scan the high-directivity beams for full angular coverage; beams are time division multiplexed and multiple TRX chains can reduce the required time for one blanket-scan. It has been observed that full beamformed channel matrix (a.k.a. profile matrix) collection is time-consuming, especially if Station-side beamforming is applied as well. In addition, channel sparsity implies that the number of meaningful elements in the profile matrix is limited. A mechanism for provisioning of measurement pilots for beam alignment and for channel state information collection is sought.
Typical channel variation sources for the backhaul link in mmWave backhaul systems include: 1) mounting pole movement due to wind sway; 2) partial blockage of first Fresnel zone; 3) environmental changes (temperature, air density). Specifically, wind sway is the main variation source for the backhaul link as compared to other variation sources. Although wind-induced channel variation can be tackled by increasing the time-domain density of measurement pilots, it may result in over-designed pilot density. Furthermore, pilot transmission is accompanied by whole bandwidth allocation, the addition pilots may not be used efficiently with high overhead. A mechanism for reducing channel variation with beamwidth adaptation is sought.
In mmWave backhaul environment, static stations render the importance of mobility support. For wireless communication systems with high directivity transmission, e.g., mmWave system, FDM among users may not be possible. Due to narrow beam nature, users with similar spatial signature is not a usual case. Users are more often served in TDM manner with whole BW. Small packet transmission is thus extremely inefficient. For power and resource saving, it is sensible to deactivate STA's backhaul link whenever possible. With the existing RAN mechanism, STA switching between RRC Idle and Connected states for power and resource saving. However, such transition is time and resource consuming, from the perspective of high-directivity transmission, e.g., mmWave system. First, contention-based random access may include conflict resolution. Second, establishing RRC connection requires several rounds of signaling between Hub and STA. Third, each transmission is provided with whole BW. A mechanism to activate/deactivate STA backhaul link efficiently is sought.